1. Field of the Invention
The invention relates to a method for optimizing normalized image log slope (NILS) of exposed lines of a lithography system, and more particularly, to a method for optimizing NILS of exposed lines by adding assist patterns into a photomask layout.
2. Description of the Prior Art
The lithographic process is a very important step in semiconductor fabrication that transfers the layout of a designed integrated circuit onto a semiconductor wafer. Conventionally, wafer producers design several photomask layouts according to IC layouts to fabricate each of the photomask layouts in a photomask. Then, lithographic and development technologies are used to proportionally transfer the circuit design pattern onto a photoresist layer on the surface of the semiconductor wafer.
As the complexity and the integration of semiconductor circuits increases, the size of the circuit design pattern on the photoresist layer decreases. However, the critical dimensions (CDs) of the pattern on the photoresist layer are limited by the resolution limit of the optical exposure tool. As a result, optical proximity effects (OPE) easily occur in the lithographic process during the formation of highly integrated circuit design patterns on a semiconductor wafer, resulting from overexposure or underexposure and causing a loss of resolution. In other words, optical proximity effects cause a difference between the pattern transferred onto the photoresist layer and the original design pattern.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a lithography system 10 according to the prior art. The lithography system 10 comprises a scanner system 12 and a semiconductor wafer 24. The scanner system 12 includes a light source 14 positioned near a top region of the scanner system 12, a first lens 16, an aperture plate 18, a second lens 20, and a photomask 22. Light beams from the light source 14 pass through the first lens 16, the aperture plate 18, the second lens 20, and the photomask 22 to the die region 26 to define a pattern of the photomask 22 on the photoresist layer on the wafer 24. The aperture plate 18 is an application of the off-axis illumination (OAI) method, which has one or several apertures that can block vertical incident light and adjust the incident angles of transmitting lights for improving the resolution and depth of focus (DOF) of the lithography process. In order to increase the integration of semiconductor products unceasingly and maintain accuracy at the same time, manufactures have undertaken research and development to refine the aperture pattern of the aperture plate 18 so as to improve the transferring performance by a common light source, such as the light with a wavelength of 248 nm, and the equipment on hand. Among the several kinds of aperture patterns, Quasar 90 is a pattern with four sector apertures, which has an included angle of 90 degrees from the horizontal axis. In contrast to other aperture patterns, such as an annular aperture, Quasar 90 can improve the array CD of the lithography process.
Please refer to FIG. 2. FIG. 2 is a chart of NILS versus space of a photomask layout with a line width of 90 nanometers (nm), which shows the NILS value with various spaces of each aperture. The NILS values represent the performance of the lithography process, and greater NILS values mean the process window of the lithography process is better. Generally speaking, when the wavelength of light source is 248 nms, NILS is required to be more than 1.3–1.5 to form effective patterns on the photoresist layer. As shown in FIG. 2, although the Quasar 90 can improve the array CD of the lithography process, its NILS is less than 1.5 (as shown with the circular mark) when the line width and space are 90 nm and 180 nm–250 nm respectively. These paremeters are less than those of other aperture patterns and do not match the limitation of NILS when the light has a wavelength of 248 nm. In this situation, the problem of forbidden pitch occurs and the transferred pattern has serious defects.
Therefore, how to raise NILS value of patterns with the space of 180–250 nm for avoiding forbidden pitch in a lithography system with Quasar 90 illumination to maintain a better array CD is still an important issue in manufactures.